Stereo perception results from the detection of stereoscopic parallax and for photogrammetric data reduction is based on the measurement of this parallax difference from stereo photography. Automated measurement of stereoscopic parallax with photogrammetric instruments requires the ability to match corresponding images and the ability to measure the parallax automatically. This corresponding or conjugate image matching involves examination of the similarity of the image structures. One type of examination is image correlation. In general, the two dimensional function representing correlation of images on transparencies T.sub.1 and T.sub.2 has the form: ##EQU1## WHERE T.sub.1 (X, Y) and T.sub.2 (X, Y) are the intensity transmittances between the two transparencies and X.sub.0 and Y.sub.0 represent the relative displacement between the transparencies. Over the last several decades, many techniques of image matching using correlation have been proposed and demonstrated. For example, in automated electronic stereo compilation, the pictoral information is converted to time dependent electrical signals and the correlation is performed in the one-dimensional time domain; see Manual of Photogrammetry, American Society of Photogrammetry, edited by M. M. Thompson (1966), as well as Johnston U.S. Pat. No. 3,636,254 and Hobrough U.S. Pat. No. 3,432,674. Other prior art optical correlation techniques are disclosed in "Coherent Optics in Mapping" by Balasubramian and Leighty (ED) found in SPIE Proceedings, Volume 45 (1974).
Notwithstanding these prior art techniques, the need exists for a correlation system with increased signal to noise ratio, greater spatial resolution, and increased speed of operation. It is therefore an object of the present invention to provide such an optical correlation technique which, in addition to providing increased signal to noise ratio, greater spatial resolution, and increased speed of operation, as compared to other prior art correlation techniques, also:
(a) determines similarity between different regions on two transparencies in a common image plane which allows the two transparencies to be individually adjusted for changes in scale, tip and tilt during the projection process;
(b) provides a normalized correlation coefficient which is independent of the individual density levels of the two transparencies and therefore also avoids problems of spurious correlation peaks;
(c) detects correlation between two transparencies as an amplitude of a single frequency ac signal, thus improving the signal to noise ratio;
(d) provides a correlation output representing the correlation between the amplitude transmittance rather than intensity transmittance, thus increasing the effective dynamic range of the image structure information that can be employed during the correlation process;
(e) allows the transparencies to be employed on film rolls or plates;
(f) is capable of operating with only partially coherent light sources, i.e., an intense monochromatic light source is required and not a fully coherent light source; and
(g) employs substantially a single imaging system with the corresponding ability to maintain geometric fidelity.